Flexible laminates are used in the electronics industry as the base materials for fabricating a wide variety of flexible interconnection products such as flexible circuit boards and flex-rigid circuit boards. Flexible circuit boards and flex-rigid circuit boards are used in notebook computers, printers and hard disc drives, as well as numerous medical devices and consumer products. Flexible laminates are also used for certain advanced applications such as chip-on-flex and fine-line circuit boards. With the electronics industry moving toward thinner, lighter, flexible and more functional products, the demand for flexible laminates continues to increase.
Adhesive based flexible laminates have been used for many of the foregoing applications. However, disadvantages associated with the use of an adhesive layer in the laminate has prohibited such laminates from being accepted for many advanced or high performance flexible circuit applications. Over the years, attempts have been made to eliminate the adhesive layer and to replace it with an adhesiveless product. These include the following approaches (1) cast polyimide on copper foil, (2) high temperature lamination of copper foil with a polyimide substrate and a polyimide adhesive, and (3) direct metallization on a polyimide film followed by electroplating.
Direct metallization has been the most promising of these approaches because of the possibility of metallizing and electroplating on both sides and the advantage of reel-to-reel processing. However, the performance characteristics of laminates made by this technique have not always been completely acceptable. For example, these laminates have usually lacked one or more of the following properties: (1) good initial adhesion, (2) good adhesion after thermal or chemical exposure, (3) an etchable tie coat layer, (4) good dimensional stability, and/or (5) uniform adhesion. Thus, there is a need for improved adhesiveless flexible laminates.
U.S. Pat. No. 4,863,808 discloses a laminate which includes a polyimide film, a layer of metallic chromium vapor deposited on the polyimide, and a layer of copper electrodeposited on the chromium layer.
U.S. Pat. No. 5,112,462 discloses a layered film structure having a metal layer securely bonded to a film layer. The laminate contains a metal-oxide attachment structure between the film and metal layer comprising randomly distributed regions of metal-oxide. The reference indicates that these laminates can be used in the manufacture of flexible printed circuit boards.
U.S. Pat. No. 5,484,517 discloses a method for forming a multi-element, thin hot film sensor on a polyimide film. The sensor is formed by first cleaning one surface of the polyimide using ultrasonic cleaning in a hot solution (i.e., at least 180.degree. F.) of deionized water and biodegradable detergent. Then, under a continuous vacuum, the surface is simultaneously cleaned by ion bombardment using an ion beam gun while nickel is deposited by evaporation. The ion beam cleaning is discontinued and copper is then deposited to an initial thickness of 1000 angstroms by evaporation without a break in the vacuum. The vacuum is then removed and a final thickness of copper is deposited by plating. Sensor patterns are then defined in the nickel and copper layers using conventional photolithography and etching techniques.